At least 75 fighters trained by U.S., British and Turkish forces have entered northern Syria, a monitoring group said.
The fighters crossed over from Turkey on Friday and Saturday and are now located in areas north of the city of Aleppo, said the London-based Syrian Observatory for Human Rights.
Their arrival comes shortly after U.S. officials acknowledged that the Pentagon's program to train and equip moderate Syrian rebels to help fight ISIS had fallen far short of its original aims.
A top U.S. general told the Senate last week that of the thousands of fighters the U.S. military was supposed to train in the first year, only four or five remained in place on the battlefield.
In light of the stark shortcomings, the Pentagon is expected to carry out a major overhaul of the training program for the rebels.
A review of the effort has been underway since an initial group of some 54 rebels put into northern Syria this summer came under attack and are no longer a functioning fighting force. That attack demonstrated that units have to be larger so the forces can protect themselves, officials said.
The fighters who entered Syria in recent days have been supplied with four-wheel-drive vehicles mounted with machine guns and ammunition, the Syrian Observatory for Human Rights said.
Warplanes from a U.S.-led coalition are carrying out a bombing campaign against ISIS positions in Syria and Iraq. But Western nations haven't sent ground troops to fight against the militant group.